Recent evidence indicates that human hematopoietic stem cell properties can be found among cells lacking CD34 and lineage commitment markers (CD34−Lin−). A major barrier in the further characterization of human CD34− stem cells is the inability to detect this population using in vitro assays because these cells only demonstrate hematopoietic activity in vivo. Using cell surface markers AC133 and CD7, subfractions were isolated within CD34−CD38−Lin− and CD34+CD38−Lin− cells derived from human cord blood. Although the majority of CD34−CD38−Lin− cells lack AC133 and express CD7, an extremely rare population of AC133+CD7− cells was identified at a frequency of 0.2%. Surprisingly, these AC133+CD7− cells were highly enriched for progenitor activity at a frequency equivalent to purified fractions of CD34+ stem cells, and they were the only subset among the CD34−CD38−Lin− population capable of giving rise to CD34+ cells in defined liquid cultures. Human cells were detected in the bone marrow of non-obese/severe combined immunodeficiency (NOD/SCID) mice 8 weeks after transplantation of ex vivo–cultured AC133+CD7− cells isolated from the CD34−CD38−Lin− population, whereas 400-fold greater numbers of the AC133−CD7− subset had no engraftment ability. These studies provide novel insights into the hierarchical relationship of the human stem cell compartment by identifying a rare population of primitive human CD34− cells that are detectable after transplantation in vivo, enriched for in vitro clonogenic capacity, and capable of differentiation into CD34+ cells.

The chemokine stromal derived factor-1␣ (SDF-1␣) has been implicated recently in the chemotaxis of primitive human hematopoietic cells, suggesting that pluripotent human stem cells express the SDF-1␣ receptor, CXCR4. By using flow cytometry and confocal microscopy, we have identified and isolated primitive subsets of human CXCR4 ؉ and CXCR4 ؊ cells. Distinctions in the progenitor content and response to SDF-1␣ in vitro indicate that CXCR4 ؉ and CXCR4 ؊ cells represent discrete populations of primitive blood cells. The i.v. transplantation of these subfractions into immune-deficient mice established that both possess comparable engraftment capacity in vivo. Human myeloid, lymphoid, and primitive CD34 ؉ CXCR4 ؉ cells were present in chimeric animals transplanted with either subset, indicating that CXCR4 ؉ and CXCR4 ؊ stem cells have equivalent proliferative and differentiative abilities. Our study indicates that the human stem cell compartment is heterogeneous for CXCR4 expression, suggesting that the relationship between CXCR4 expression and stem cell repopulating function is not obligatory.stromal-derived factor-1␣ ͉ xenotransplantation

SummaryThe INR system was developed to standardize PT reporting in patients on oral anticoagulants. We prospectively collected blood samples from 29 patients with liver impairment (INR 1.5-3.5). Control patients were on warfarin (n = 31). PT’s were measured on an ACL-300 with three thromboplastin reagents. INR’s were calculated using instrument specific ISI’s. Other tests performed were FDP’s, fibrinogen, aPTT, factors II, V, VII and X. The INR’s for each patient in the study population using the three thromboplastin reagents were significantly different (p = 0.0001). Those for the control population were not (p = 0.0658). Fibrinogen, factors V, II and X were different at the 5% level of significance between the populations. FDP’s were detected in 17 study subjects. The INR system is not valid for comparison of patients with liver impairment because different reagents do not give the same INR for the same sample. It is, however, no less valid than the use of PT with different thromboplastin reagents. Further study is recommended.